1. Field of the Invention
The present invention is directed to a method and apparatus to reduce the concentration of dissolved species in water and, more specifically, to a method and apparatus for microfiltration.
2. Description of the Related Art
Water may contain a variety of materials including, among other contaminants, divalent cations that increase the xe2x80x9chardnessxe2x80x9d of the water. The most prevalent divalent cations contributing to hardness are calcium and magnesium. Hard water may present a variety of problems for users in residential or industrial use. In addition, due to their negative solubility coefficients, calcium and magnesium in the water may precipitate out and form scale on heat exchange surfaces. Elevated levels of hardness may make it more difficult to remove other undesirable components such as other ionic compounds or organic compounds and may foul water treatment systems. For example, reverse osmosis membranes may be fouled by even relatively low levels of hardness.
Several techniques exist for the reduction of water hardness. For instance, calcium and magnesium cations may be removed by ion exchange where calcium and magnesium ions are exchanged for alternative cations that have a less deleterious effect in the water. Such methods require a source of alternative cations and are limited by the relative affinity of various ion exchange substrates to the competing cations as well as by the kinetics of the ion exchange itself. After use, these ion exchange substrates, for example, ion exchange resins, must be either recharged or replaced.
Calcium, magnesium and silica may also be removed by precipitation or xe2x80x9clime softening.xe2x80x9d Generally, either lime or soda ash, or both, is added to the water to reduce the amounts of calcium, magnesium and silica that are in solution to about the level of solubility of calcium carbonate and magnesium hydroxide, respectively. The precipitated matter is then removed from the water either by filtration or by settling.
Traditionally, lime softening involves the conversion of soluble salts in water into non-soluble salts. In some non-critical applications, there is no need to separate the sediments from the water. In more critical applications, separation and removal of the non-soluble salts is mandatory. Therefore, separation processes, as well as chemical processes, may be involved in lime water softening.
A conventional sedimentation following flocculation and agglomeration is currently employed as the most common method of separation.
A significant improvement of the separation technique was achieved by using membranes as a separation media. In 1983, D. Comstock, et al. published results of the xe2x80x9cHydropermxe2x80x9d process using cross flow microfiltration as a separation device after the lime softening process. Kedem, et al., in U.S. Pat. No. 5,152,904, describe water treated by the addition of sodium hydroxide being contacted with large excess of specific-sized calcium carbonate crystals followed by separation.
The conventional methods of water softening, however, suffer from high operating and capital cost, limited throughput, or insufficient reduction in concentrations of various ions such as calcium, magnesium or silica.
The present invention provides a method and apparatus for microfiltration. The method uses cross-flow microfiltration through sub-micron filtration media to reduce the level of, for example, calcium and magnesium, to concentrations that may be below that which is predicted by the solubility limits of calcium and magnesium salts under similar conditions.
In one embodiment, the method of the invention includes passing water including a precipitate through a substantially tubular semipermeable membrane. The membrane is comprised of sub-micron filtration media. The filtrate is then collected from the outer surface of the membrane, wherein the filtrate contains concentrations of dissolved precipitate at levels below the solubility limit of the precipitate.
In another embodiment, the apparatus of the invention includes at least one substantially tubular membrane having an inner wall and an outer wall, the inner wall defining a lumen. An inlet is in fluid communication with the lumen of the membrane. A momentary dynamic membrane including calcium carbonate is disposed on a portion of the inner wall of the membrane. A first outlet is in fluid communication with the lumen of the membrane, and a second outlet is in fluid communication with the outer wall of the membrane.
In another embodiment, the invention includes a method of reducing the concentration of divalent cations in an aqueous fluid. The method includes adding an anion to an aqueous fluid at a concentration adequate to precipitate out at least some dissolved cations in the form of a salt. The fluid containing precipitated salt is then passed along a semipermeable substantially tubular membrane. Aqueous fluid is then collected outside of the substantially tubular membrane, wherein the aqueous fluid contains dissolved divalent cations at a molar concentration below the molar solubility limit of the salt.
In another embodiment, the invention is directed to a method of pretreating water. The method includes adding an anion to the water at a level adequate to precipitate out at least a portion of any cations present in the water. The water is then passed through a substantially tubular membrane at turbulent flow. Filtrate is then collected from the substantially tubular semipermeable membrane.
In another embodiment, the invention is directed to a method of pretreating water. The method includes adding a cation to the water at a level adequate to precipitate out at least a portion of any anions present in the water. The water is then passed through a substantially tubular semipermeable membrane at turbulent flow. Filtrate is then collected from the substantially tubular semipermeable membrane.